Vehicle emission control systems may be configured to store refueling vapors, and in some examples running-loss vapors and diurnal emissions in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy for the vehicle. In a typical canister purge operation, a canister purge valve (CPV) coupled between the engine intake and the fuel vapor canister is opened or duty cycled, allowing for intake manifold vacuum to be applied to the fuel vapor canister. Fresh air may be drawn through the fuel vapor canister via an open canister vent valve. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
Certain hybrid electric vehicles, for example plug-in hybrid electric vehicles (PHEVs) further include a fuel tank that is sealed via a fuel tank isolation valve (FTIV). Such fuel tanks are sealed in order to reduce loading of the fuel vapor canister during diurnal temperature fluctuations and while the vehicle is in operation, as opportunities for purging of the fuel vapor canister may be limited due to limited engine run-time for such vehicles. While such fuel tanks may reduce canister loading, pressure builds within such fuel tanks may have to be periodically relieved for fuel tank integrity reasons and/or to reduce fuel tank depressurization times in response to requests to refuel the fuel tank. In one example, while the engine is operating to combust air and fuel vehicle control strategy may duty cycle the FTIV (with the CPV open) in order to relieve fuel tank pressure and route fuel tank vapors to the engine for combustion. However, depending on environmental (e.g. high ambient temperatures) and/or vehicle operating conditions (e.g. fuel slosh event(s) due to vehicle motion), an amount of vapors inducted to the engine during a fuel tank pressure control strategy may undesirably result in engine stability issues (e.g. engine hesitation and/or engine stall). In response to an indication of engine stability degradation during fuel tank pressure control, purge control strategy and fuel tank pressure control strategy may be discontinued. While such action may avoid engine hesitation and/or stall, such action may disrupt purging and/or fuel tank pressure control, which may lead to increased depressurization times in response to refueling requests and/or an increase in undesired evaporative emissions due to inefficient purging of the fuel vapor canister. Such issues may be particularly exacerbated in hybrid vehicles such as Start/Stop (S/S) vehicles where engine run time is limited.
The inventors herein have recognized the above-mentioned issues, and have herein developed systems and methods to at least partially address them. In one example, a method comprises reducing a pressure in a fuel tank by routing vapors from the fuel tank through a portion of a fuel vapor canister positioned in an evaporative emissions system of a vehicle and not through an entirety of the fuel vapor canister. In response to an indication of a condition of degraded stability of an engine of the vehicle, the method may include re-routing the vapors from the fuel tank through the entirety of the fuel vapor canister. In this way, a rate at which the fuel vapors are inducted into the engine may be reduced due to the fuel vapors passing across a greater amount of adsorbent material within the canister, which may thus mitigate the issue of degraded engine stability without discontinuing the operation to reduce the pressure.
As one example of the method, the portion of the fuel vapor canister may comprise a buffer region of the fuel vapor canister.
As another example of the method, routing the vapors from the fuel tank through the portion of the fuel vapor canister may further comprise routing the vapors through the portion of the fuel vapor canister and then to the engine for combustion. Alternatively, re-routing the vapors from the fuel tank through the entirety of the fuel vapor canister may further comprise routing the vapors to a vent line that couples the fuel vapor canister to atmosphere, and then through the entirety of the fuel vapor canister en route to the engine. In such an example, routing vapors through the portion of the fuel vapor canister may further comprise commanding fully open a canister vent valve positioned in the vent line without duty cycling the canister vent valve. Alternatively, re-routing the vapors through the entirety of the fuel vapor canister may further comprise duty cycling the canister vent valve at a predetermined duty cycle.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.